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United States Patent |
5,679,384
|
Emoto
|
October 21, 1997
|
Injection apparatus for an electric injection molding machine
Abstract
An injection apparatus for an electric injection molding machine includes a
drive unit case, a metering motor, an injection motor disposed to be
coaxial with the metering motor, a first drive force transmission
mechanism connected between the metering motor and a screw, being adapted
to prevent relative rotational movement while allowing relative axial
movement, a motion conversion mechanism for converting rotational movement
to linear movement so as to advance the screw, a second drive force
transmission mechanism for preventing relative rotational movement while
allowing relative axial movement, and a third drive force transmission
mechanism for preventing relative axial movement while allowing relative
rotation. Since it is unnecessary to use timing belts for advancing and
retracting movement and rotational movement of the screw, no noise is
generated during drive, and maintenance and management are facilitated. In
addition, accuracy in controlling the speed, position and the like of the
screw is enhanced.
Inventors:
|
Emoto; Atsushi (Chiba, JP)
|
Assignee:
|
Sumitomo Heavy Industries, Ltd. (JP)
|
Appl. No.:
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591794 |
Filed:
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January 25, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
425/145; 264/40.7; 425/149; 425/167; 425/542 |
Intern'l Class: |
B29C 045/77 |
Field of Search: |
425/145,149,167,542
264/40.7
|
References Cited
U.S. Patent Documents
4755123 | Jul., 1988 | Otake | 425/145.
|
4879077 | Nov., 1989 | Shimizu et al. | 425/145.
|
5129808 | Jul., 1992 | Watanabe et al. | 425/145.
|
5332382 | Jul., 1994 | Kasai et al. | 425/145.
|
Primary Examiner: Heitbrink; Tim
Attorney, Agent or Firm: Lorusso & Loud
Claims
What is claimed is:
1. An injection apparatus for an electric injection molding machine,
comprising:
(a) an injection cylinder and an injection screw mounted within said
injection cylinder for rotational and linear motion relative to said
cylinder;
(b) a drive unit case;
(c) a metering motor mounted within said drive unit case for generating
said rotational motion;
(d) an injection motor, mounted within said drive unit case coaxial with
said metering motor, for generating rotary motion;
(e) first drive force transmission means connected between said metering
motor and said injection screw, for transmitting said rotational motion
from said metering motor to said injection screw;
(f) motion conversion means connected between said injection motor and said
injection screw, for converting said rotary motion of said injection motor
to the linear motion for advancing said screw;
(g) second drive force transmission means, connected between said drive
unit case and said injection screw, for preventing transmission of the
rotary motion of said injection motor to said injection screw while
allowing transmission of the linear motion to said injection screw; and
(h) third drive force transmission means, connected between said motion
conversion means and said injection screw, for transmitting the linear
motion from said motion conversion means to said injection screw while
allowing rotational motion of said injection screw.
2. An injection apparatus for an electric injection molding machine
according to claim 1, wherein:
said metering motor comprises a first rotor and a hollow first rotor shaft
fixed to said first rotor and rotatably supported with respect to said
drive unit case; and
said first drive force transmission means comprises a first spline nut
fixed to said first rotor shaft and a first spline shaft spline-engaged
with said first spline nut.
3. An injection apparatus for an electric injection molding machine
according to claim 1, wherein:
said injection motor comprises a second rotor and a hollow second rotor
shaft fixed to said second rotor and rotatably supported with respect to
said drive unit case; and
said motion conversion means comprises a ball screw shaft integrally
connected to said second rotor shaft and rotatably supported with respect
to said drive unit case and a ball nut screw-engaged with said ball screw
shaft for advancing and retracting linear movement responsive to rotation
of said ball screw shaft.
4. An injection apparatus for an electric injection molding machine
according to claim 1, wherein said second drive force transmission means
comprises:
a second spline nut fixed to said drive unit case; and
a second spline shaft spline-engaged with said second spline nut.
5. An injection apparatus for an electric injection molding machine
according to claim 1, further comprising an absolute pulse encoder is
provided at an end of said ball screw shaft.
6. An injection apparatus for an electric injection molding machine,
comprising:
(a) an injection cylinder and an injection screw mounted within said
injection cylinder for rotational and linear motion relative to said
cylinder;
(b) a drive unit case;
(c) a metering motor mounted within said drive unit case and including a
first stator fixed to said drive unit case and a first rotor rotatable
relative to said first stator;
(d) an injection motor mounted within said drive unit case coaxial with
said metering motor and including a second stator fixed to said drive unit
case and a second rotor rotatable relative to said second stator;
(e) a hollow first rotor shaft fixed to said first rotor and rotatably
supported with respect to said drive unit case;
(f) a hollow second rotor shaft fixed to said second rotor and rotatably
supported with respect to said drive unit case;
(g) a ball screw shaft integrally connected to said second rotor shaft and
rotatably supported with respect to said drive unit case;
(h) a ball nut screw-engaged with said ball screw shaft for advancing and
retracting linear movement responsive to rotation of said ball screw
shaft;
(i) a first spline nut fixed to said first rotor shaft;
(j) a first spline shaft spline-engaged with said first spline nut and
having a front end connected to said injection screw;
(k) a second spline nut fixed to said drive unit case; and
(l) a second spline shaft spline-engaged with said second spline nut and
having a front end and a rear end, said first spline shaft being rotatably
supported by the front end of said second spline shaft and the rear end of
said second spline shaft being connected to said ball nut.
7. An injection apparatus for an electric injection molding machine
according to claim 6, wherein said ball screw shaft, said ball nut, said
first spline shaft and said second spline shaft are removably disposed
within said first and second rotor shafts.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an injection apparatus for an electric
injection molding machine.
2. Description of the Related Art
Conventionally, in an injection molding machine, resin heated and melted in
a heating cylinder is injected into a cavity of a die under a high
pressure so that the cavity is filled with the resin. The molten resin is
then cooled and hardened to obtain a molded product. The molded product is
then taken out from the die after the die is opened.
The injection molding machine includes a die clamp apparatus and an
injection apparatus. The die clamp apparatus is provided with a stationary
platen and a movable platen, and a movable die is contacted with and
separated from a stationary die by advancing and retracting the movable
platen using a die clamping cylinder.
The injection machine includes a heating cylinder for heating and melting
resin supplied from a hopper and an injection nozzle for injecting the
molten resin. Further, a screw is disposed within the heating cylinder for
advancing and retracting movement. The screw is advanced to inject resin
and retracted to meter the resin.
An electric injection molding machine has been proposed in which electric
motors are used to advance and retract the injection apparatus and to
advance and retract the screw.
FIG. 1 is a schematic view of an injection apparatus used in a conventional
electric injection molding machine.
In FIG. 1, numeral 2 denotes an injection apparatus, and numeral 4 denotes
a frame of the injection apparatus 2. A heating cylinder 21 is fixedly
provided on the front side (left side in FIG. 1) of the frame 4, and an
injection nozzle 21a is provided at the front end (left-side end in FIG.
1) of the heating cylinder 21. A hopper 21b is disposed on the heating
cylinder 21, and a screw 20 is disposed within the heating cylinder 21
such that the screw 20 is rotatable and axially movable. The rear end
(right-side end in FIG. 1) of the screw 20 is rotatably supported by a
support member 5.
A first servomotor 6 is attached to the support member 5, and rotation of
the first servomotor 6 is transmitted to the screw 28 via a timing belt
7a.
Further, a screw shaft 8 is rotatably supported on the frame 4 in parallel
with the screw 20, and the rear end of the screw shaft 8 is connected to a
second servomotor 9 via a timing belt 7b. Therefore, the screw shaft 8 can
be rotated by the second servomotor 9. The front end of the screw shaft 8
is in screw engagement with a nut 5a fixed to the support member 5.
Accordingly, the nut 5a can be moved axially by rotating the screw shaft 8
by the second servomotor 9 via the timing belt 7b.
Next, the operation of the injection apparatus 2 having the above-described
structure will be described.
In a metering stage, the first servomotor 6 is driven to rotate the screw
20 via the timing belt 7a, thereby retracting the screw 20 by a
predetermined amount. At this time, resin supplied from the hopper 21b is
heated and melted within the heating cylinder 21, and accumulates on the
front side of the screw 20 as the screw 20 is retracted.
In a subsequent injection stage, the injection nozzle 21a is pressed
against an unillustrated die, and the second servomotor 9 is driven to
rotate the screw shaft 8 via the timing belt 7b. At this time, with the
rotation of the screw shaft 8, the support member 5 is moved to advance
the screw 20. As a result, the resin accumulated on the front side of the
screw 20 is injected from the injection nozzle 21a.
FIG. 2 is a schematic view of another injection apparatus used in a
conventional electric injection molding machine.
In FIG. 2, numeral 2 denotes an injection apparatus, and numeral 4 denotes
a frame of the injection apparatus 2. A heating cylinder 21 is fixedly
provided on the front side (left side in FIG. 2) of the frame 4, and an
unillustrated injection nozzle is provided at the front end of the heating
cylinder 21. A screw 20 is disposed within the heating cylinder 21 such
that the screw 20 is rotatable and axially movable. A ball screw 31 is
formed extending from the rear end of the screw 20 and a spline shaft 32
is formed extending from the rear end of the ball screw 31.
A through motor 34 for injection is attached to the frame 4 such that the
through motor 34 surrounds the ball screw 31, and a ball screw nut 37 is
fixed to the through motor 34. Further, a through motor 35 for metering is
disposed such that the through motor 35 surrounds the spline shaft 32, and
a spline nut 38 is fixed to the through motor 35.
A numerical controller 39 is connected to the through motor 34 for
injection and to the through motor 35 for metering. Injection and metering
are performed by selectively rotating these motors 34 and 35 by the
numerical controller 39. In detail, in a metering stage, the through motor
35 for metering and the through motor 34 for injection are simultaneously
rotated at the same speed, so that the spline shaft the ball screw 31 and
the screw 20 rotate for metering. At this time, power supplied to the
through motor 34 for injection may be adjusted to provide a difference in
rotational speed between the ball screw nut 37 and the spline nut 38,
thereby retracting the screw 20. With this operation, back pressure can be
controlled during metering.
In an injection stage, the through motor 34 for injection is rotated while
the through motor 35 for metering is stopped, so that the ball screw 31 is
advanced by rotation of the ball screw nut 37. As a result, the screw 20
is advanced to perform injection.
However, in the conventional injection apparatus shown in FIG. 1, noise is
produced by the timing belts 7a and 7b. Also, the injection apparatus is
large because the first and second servomotors 6 and 9 are not disposed on
the same axis as that of the screw 20. Further, wear of the timing belts
7a and 7b makes maintenance and management troublesome, and the elasticity
of the timing belts 7a and 7b degrades accuracy in controlling the speed,
position and the like of the screw 20. In addition, since the first
servomotor 6 is advanced and retracted together with the screw 20, the
reliability of wiring for the motor and the like becomes low.
In the conventional injection apparatus shown in FIG. 2, the ball screw 31
and the spline shaft 32 are integrated together, and the ball screw nut 37
screw-engaged with the ball screw 31 is rotated by the through motor 34
for injection, while the spline nut 38 spline-engaged with the spline
shaft 32 is rotated by the through motor 35 for metering.
When metering is performed, for example, the through motor 34 for injection
is rotated faster than the through motor 35 for metering so as to apply
back pressure to the screw 20. Therefore, the through motor 35 for
metering and the through motor 34 for injection must be driven in a
synchronized manner. However, since the through motor 35 for metering and
the through motor 34 for injection are both difficult to control, accuracy
in controlling back pressure is inadequate.
When injection is performed, the ball screw nut 37 is rotated, so that a
large rotational inertia is produced at the drive section. Therefore,
control characteristics such as acceleration are inadequate.
At this time, the through motor 35 for metering restricts rotation of the
screw 20. Therefore, the through motor 35 for metering produces a force
equal to the rotational force for injection so as to maintain its initial
position. Accordingly, the through motor 35 for metering must have a large
capacity.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above-mentioned problems
in the conventional injection apparatus for an electric injection molding
machine, and to provide an injection apparatus for an electric injection
molding machine which facilitates maintenance and management, which
enhances accuracy in controlling the speed, position and the like of a
screw, and which facilitates the control of motors.
To achieve the above object, the present invention provides an improved
injection apparatus for an electric injection molding machine. The
injection apparatus includes a drive unit case, a metering motor disposed
within the drive unit case, an injection motor disposed within the drive
unit case and coaxial with the metering motor, a first drive force
transmission means connected between the metering motor and a screw, being
adapted to prevent relative rotational movement while allowing relative
axial movement, a motion conversion means connected between the injection
motor and the screw, being adapted to convert rotational movement to
linear movement so as to advance the screw, a second drive force
transmission means connected between the drive unit case and the screw,
being adapted to prevent relative rotational movement while allowing
relative axial movement, and a third drive force transmission means
connected between the motion conversion means and the screw, being adapted
to prevent relative axial movement while allowing relative rotational
movement.
In this injection apparatus, when the injection motor is driven in an
injection stage, rotation produced by the injection motor is transmitted
to the motion conversion means, in which the rotational movement is
converted into linear movement.
At this time, the metering motor is not driven. Thus, the screw is advanced
while being prevented from rotating, so that resin accumulated on the
front side of the screw is injected from the injection nozzle.
When the metering motor is driven in a subsequent metering stage, rotation
produced by the metering motor is transmitted to the screw via the second
drive force transmission means. Accordingly, the screw is retracted with
rotation.
As described above, it is unnecessary to use timing belts for advancing and
retracting movement and rotational movement of the screw.
Therefore, no noise is generated during drive, and maintenance and
management can be facilitated. In addition, accuracy in controlling the
speed, position and the like of the screw can be enhanced.
Since the third drive force transmission means transmits linear movement
from the motion conversion means to the screw while allowing the relative
rotational movement of the screw, it is unnecessary to synchronously drive
the metering motor and the injection motor. This facilitates control of
the metering motor and the injection motor, and enhances accuracy in
controlling back pressure.
In another embodiment of an injection apparatus for an electric injection
molding machine according to the present invention, the first drive force
transmission means includes a hollow first rotor shaft fixed to the rotor
of the metering motor and rotatably supported with respect to the drive
unit case, a first spline nut fixed to the first rotor shaft, and a first
spline shaft spline-engaged with the first spline nut.
In still another embodiment of an injection apparatus for an electric
injection molding machine according to the present invention, the motion
conversion means includes a hollow second rotor shaft fixed to the rotor
of the injection motor and rotatably supported with respect to the drive
unit case, a ball screw shaft integrally connected to the second rotor
shaft and rotatably supported with respect to the drive unit case, and a
ball nut screw-engaged with the ball screw shaft, being adapted to be
advanced and retracted with rotation of the ball screw shaft.
In yet another embodiment of an injection apparatus for an electric
injection molding machine according to the present invention, the second
drive force transmission means includes a second spline nut fixed to the
drive unit case, and a second spline shaft spline-engaged with the second
spline nut.
Also, the present invention provides an injection apparatus for an electric
injection molding machine which includes a drive unit case, a metering
motor disposed within the drive unit case and including a stator fixed to
the drive unit case and a rotor rotatable relative to the stator, an
injection motor disposed within the drive unit case to be coaxial with the
metering motor and including a stator fixed to the drive unit case and a
rotor rotatable relative to the stator, a hollow first rotor shaft fixed
to the rotor of the metering motor and rotatably supported with respect to
the drive unit case, a hollow second rotor shaft fixed to the rotor of the
injection motor and rotatably supported with respect to the drive unit
case, a ball screw shaft integrally connected to the second rotor shaft
and rotatably supported with respect to the drive unit case, a ball nut
screw-engaged with the ball screw shaft, being adapted to be advanced and
retracted with rotation of the ball screw shaft, a first spline nut fixed
to the first rotor shaft, a first spline shaft spline-engaged with the
first spline nut and connected to a screw through its front end, a second
spline nut fixed to the drive unit case, and a second spline shaft
spline-engaged with the second spline nut, the first spline shaft being
rotatably supported by the front end of the second spline shaft and the
rear end of the second spline shaft being connected to the ball nut.
When the injection motor is driven in an injection stage so as to rotate
the rotor of the injection motor, rotation of the rotor of the injection
motor is transmitted to the ball screw shaft via the second rotor shaft.
Due to the rotation of the ball screw shaft, a thrust force is generated
in the ball nut, whereby the ball nut is advanced.
At this time, the metering motor is not driven and the rotor of the
metering motor therefore is in a stopped state. Accordingly, the first
spline shaft is advanced without being rotated, so that the screw is
advanced. As a result, resin accumulated on the front side of the screw is
injected from the injection nozzle.
When the metering motor is driven in a subsequent metering stage so as to
rotate the rotor of the metering motor, rotation of the rotor of the
metering motor is transmitted to the first spline shaft via the first
rotor shaft. The rotation of the first spline shaft is then transmitted to
the screw. As a result, the screw is retracted while being rotated. At
this time, the injection motor is rotated in a direction for retracting
the screw while controlling the back pressure of the resin to be metered.
As described above, it is unnecessary to use timing belts for advancing and
retracting movement and rotational movement of the screw.
Therefore, no noise is generated during driving of the screw, and
maintenance and management are facilitated. In addition, accuracy in
controlling the speed, position and the like of the screw is enhanced.
Since the first spline shaft and the ball nut can rotate relative to each
other, it is unnecessary to synchronously drive the metering motor and the
injection motor. This facilitates control of the metering motor and the
injection motor, and enhances accuracy in controlling back pressure.
Moreover, since the rotational force of the injection motor is resisted by
the second spline nut, the metering motor does not receive the rotational
force. Accordingly, the capacity of the metering motor can be decreased.
Since both the metering motor and the injection motor remain in a fixed
position in all stages, the reliability of wiring for the motors and the
like is enhanced.
In yet another embodiment of an injection apparatus for an electric
injection molding machine according to the present invention, the ball
screw shaft, the ball nut, the first spline shaft and the second spline
shaft are removably disposed within the first and second rotor shafts.
In this case, the drive unit can be separated into a motor assembly and a
drive shaft assembly. Therefore, it is possible to separately manufacture
the motor assembly and the drive shaft assembly. After that, the metering
motor and the injection motor of the motor assembly are driven to
respectively check their operations. After the checking, the drive shaft
assembly is inserted into the motor assembly so as to check the operation
of the drive unit.
Therefore, the motor assembly and the drive shaft assembly can be
maintained and managed separately.
Yet another embodiment of an injection apparatus according to the present
invention includes an absolute pulse encoder provided at an end of the
ball screw shaft.
In this case, the position of the ball nut is calculated based on an
absolute rotational position signal output from the absolute pulse encoder
and the lead of the ball screw shaft so as to control the position of the
screw.
Therefore, the position of the ball nut can be detected by the absolute
pulse encoder, and the position, speed and the like of the screw can be
controlled. As a result, it becomes unnecessary to provide an encoder for
driving the injection motor. This reduces the cost of the injection
apparatus.
In addition, since a mechanism for transmitting drive power is not
interposed between the rotor of the injection motor and the ball screw
shaft, accuracy in detecting the position of the ball nut is enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure and features of the injection apparatus for an electric
injection molding machine according to the present invention will be
readily appreciated as the same becomes better understood by referring to
the accompanying drawings, in which:
FIG. 1 is a schematic view of an injection apparatus used in a conventional
electric injection molding machine;
FIG. 2 is a schematic view of another injection apparatus used in a
conventional electric injection molding machine;
FIG. 3 is a schematic view of a heating cylinder of an electric injection
molding machine according to an embodiment of the present invention;
FIG. 4 is a sectional view of a drive unit of an injection apparatus
according to the embodiment of FIG. 3; and
FIG. 5 is an exploded view of the drive unit of the injection apparatus
according to the embodiment of FIG. 3.
DESCRIPTION OF PREFERRED EMBODIMENT
An embodiment of the present invention will next be described in detail
with reference to the drawings.
In FIG. 3, numeral 12 denotes a heating cylinder, which has an injection
nozzle 12a at its front end (left-side end in FIG. 3). A screw 22 is
disposed within the heating cylinder 12 such that the screw 22 is allowed
to advance, retract and rotate.
The screw 22 has a screw head 22a at its front end and extends rearward (in
the rightward direction in FIG. 3) within the heating cylinder 12. The
rear end (right-side end in FIG. 3) of the screw 22 is connected to a
drive unit, which will be described later. Also, a spiral flight 23 is
formed on the peripheral surface of the screw 22, resulting in the
formation of a groove 26.
A resin supply port 29 is formed at a predetermined position on the heating
cylinder 12, and a hopper 30 is fixed to the resin supply port 29. The
resin supply port 29 is formed at a position such that the resin supply
port 29 faces the rear end portion of the groove 26 when the screw 22 is
positioned at a forwardmost position (in the leftward direction in FIG. 3)
of the heating cylinder 12.
In a metering stage, the drive unit is driven to retract the screw 22 with
rotation. With this operation, pelleted resin 33 in the hopper 30 descends
and enters the heating cylinder 12. The resin 33 is then advanced along
the groove 26.
An unillustrated heater is disposed around the heating cylinder 12. The
heating cylinder 12 is heated by the heater so as to melt the resin 33 in
the groove 26. Accordingly, when the screw 22 is retracted by a
predetermined amount while being rotated, molten resin 33 for one shot
accumulates on the front side of the screw head 22a.
In a subsequent injection stage, the screw 22 is advanced without rotation
by driving the drive unit. With this operation, the resin 33 accumulated
on the front side of the screw head 22a is injected from the injection
nozzle 12a, and is charged into a cavity of an unillustrated die.
Next, the driving unit will be described with reference to FIGS. 4 and 5.
In FIG. 4, numeral 11 denotes a drive unit case enclosing the drive unit,
and the drive unit case 11 is fixed to the rear end (right-side end in
FIG. 4) of the heating cylinder 12. The drive unit case 11 comprises a
front cover 13, a front support 14, a center support 15, a rear support
16, a rear cover 17, a front frame 41 connecting the front support 14 and
the center support 15, and a rear frame 42 connecting the center support
15 and the rear support 16. The front cover 13 is fixed to the front
support 14 using bolts 18, and the rear cover 17 is fixed to the rear
support 16 using bolts 19.
A metering motor 44 and an injection motor 45 are disposed at front and
rear portions of the drive unit case 11, respectively, such that the
metering motor 44 and the injection motor 45 share a common rotational
axis. The metering motor 44 includes a stator 46 fixed to the front frame
41, and a rotor 47 disposed inside the stator 46. The injection motor 45
includes a stator 48 fixed to the rear frame 42, and a rotor 49 disposed
inside the stator 48.
The rotor 47 is supported so as to be rotatable relative to the drive unit
case 11. In detail, a hollow first rotor shaft 56 is fitted into the rotor
47 and fixed thereto, and the front end of the first rotor shaft 56 is
supported by the front support 14 via a bearing 51 while the rear end of
the first rotor shaft 56 is supported by the center support 15 via a
bearing 52.
Similarly, the rotor 49 is supported so as to be rotatable relative to the
drive unit case 11. In detail, a hollow second rotor shaft 57 is fitted
into the rotor 49 and fixed thereto, and the front end of the second rotor
shaft 57 is supported by the center support 15 via a bearing 53 while the
rear end of the second rotor shaft 57 is supported by the rear support 16
via a bearing 54.
When a current of a predetermined frequency is supplied to the stator 46 of
the metering motor 44, the screw 22 (FIG. 3) is retracted while being
rotated. To realize this motion, a first spline nut 62 is fixed to the
inner circumferential of the front end of the first rotor shaft 56, the
first spline nut 62 and a first spline shaft 63 are spline-engaged with
each other, and the screw 22 is fixed to the front end of the first spline
shaft 63. The first spline nut 62 and the first spline shaft 63 constitute
a first drive force transmission means which restricts relative rotational
movement and allows relative axial movement. The first spline shaft 63 has
a length corresponding to the stroke of the screw 22.
Accordingly, when the rotor 47 is rotated by driving the metering motor 44,
the rotation of the rotor 47 is transmitted to the screw 22 via the first
rotor shaft 56, the first spline nut 62, and the first spline shaft 63, so
that the screw 22 is rotated. As a result, the resin 33 is advanced along
the groove 26 while being gradually melted, and the screw 22 is retracted
due to back pressure which is generated with the advancement of the resin
33.
Since the first spline nut 62 and the first spline shaft 63 are
spline-engaged with each other, the first spline shaft 63 is retracted
relative to the first spline nut 62.
Also, the screw 22 can be advanced without rotation by supplying the stator
48 of the injection motor 45 with a current of a predetermined frequency.
To realize this motion, an annular bearing retainer 64 is fixed to the
rear end of the second rotor shaft 57, and the ball screw shaft 65 is
fitted into the central opening of the bearing retainer 64 and fixed
thereto. The ball screw shaft 65 is supported to be rotatable with respect
to the drive unit case 11. In detail, the ball screw shaft 65 is supported
by the bearing 66 via the bearing retainer 64 and is also supported by the
rear cover 17 via a bearing 67 disposed on the rear side of the bearing
66.
A ball nut 69 is disposed within the second rotor shaft 57 in an axially
movable manner. The ball nut 69 is screw-engaged with the ball screw shaft
65 to constitute a motion conversion means. Accordingly, rotation of the
rotor 49 is transmitted to the ball screw shaft 65 via the second rotor
shaft 57 and the bearing retainer 64, so that the rotational movement of
the rotor 49 is converted into linear movement to advance and retract the
ball nut 69.
To prevent the ball nut 69 from rotating together with the ball screw shaft
65, a second spline shaft 71 is fixed to the front end of the ball nut 69,
and a second spline nut 76 fixed to the center support 15 is
spline-engaged with the second spline shaft 71. The second spline nut 76
and the second spline shaft 71 constitute a second drive force
transmission means which restricts relative rotational movement and allows
relative axial movement. The second spline shaft 71 has a length
corresponding to the stroke of the screw 22.
A bearing box 72 serving as a third drive force transmission means is fixed
to the front end of the second spline shaft 71. A thrust bearing 73 is
disposed in the front end portion of the bearing box 72 while a bearing 74
is disposed in the rear end portion of the bearing box 72. In this case,
the bearing box 72 restricts relative axial movement and allows relative
rotational movement. Accordingly, the first spline shaft 63 is supported
by the thrust bearing 73 and the bearing 74 while being allowed to rotate
relative to the second spline shaft 71 and the ball nut 69. Numeral 85
denotes an absolute pulse encoder for detecting the position of the screw
22, and numeral 86 denotes a bracket for supporting the absolute pulse
encoder 85.
Next, the operation of the drive unit having the above-described structure
will be described.
When a current is supplied to the stator 48 of the injection motor 45 in an
injection stage, the rotor 49 is rotated, and the rotation of the rotor 49
is transmitted to the ball screw shaft 65 via the second rotor shaft 57
and the bearing retainer 64, so that the ball screw shaft 65 is rotated.
Since the second spline nut 76 fixed to the center support 15 is
spline-engaged with the second spline shaft 71, the ball nut 69 does not
rotate. Accordingly, a thrust force is generated in the ball nut 69
whereby the ball nut 69 is advanced.
During the above-described injection stage, the metering motor 44 is not
driven, and the rotor 47 is therefore in a stopped state. Accordingly, the
first spline shaft 63 disposed on the front side of the ball nut 69 is
advanced without being rotated so as to advance the screw 22.
As described above, rotational movement produced by the injection motor 45
is converted into linear movement by the ball screw shaft 65 and the ball
nut 69. As a result, the resin 33 accumulated on the front side of the
screw 22 is injected from the injection nozzle 12a.
When a current is supplied to the stator 46 of the metering motor 44 in a
subsequent metering stage, the rotor 47 is rotated, and the rotation of
the rotor 47 is transmitted to the first spline shaft 63 via the first
rotor shaft 56 and the first spline nut 62, so that the first spline shaft
63 is rotated. The rotation of the first spline shaft 63 is then
transmitted to the screw 22 so as to rotate the screw 22. With the
rotation of the screw 22, the resin 33 is advanced along the groove 26
while being gradually melted, and the screw 22 is retracted due to back
pressure which is generated with the advancement of the resin 33.
Since the first spline nut 62 and the first spline shaft 63 are
spline-engaged with each other, the first spline shaft 63 is retracted
relative to the first spline nut 62.
The injection motor 45 is driven while controlling the back pressure of the
resin 33 to be metered, and the rotor 49 is rotated in a direction for
retracting the screw 22. The back pressure can be obtained based on, for
example, the load applied to the screw 22 and so on in the axial direction
which is detected by an unillustrated load sensor, or the pressure of the
resin BB within the heating cylinder detected by an unillustrated pressure
sensor.
As described above, since the timing belts 7a and 7b (see FIG. 1) are not
required for advancing and retracting movement and rotational movement of
the screw 22, no noise is generated during drive of the screw, and
maintenance and management are facilitated. In addition, accuracy in
controlling the speed, position and the like of the screw 22 are enhanced.
Moreover, since the ball screw shaft 65 is rotated during injection,
rotational inertia is decreased compared to the case where the ball nut 69
is rotated for injection. This improves control characteristics such as
acceleration.
Since the first spline shaft 63 and the ball nut 69 can rotate relative to
each other via the thrust bearing 73 and the bearing 74, it is unnecessary
to synchronously drive the metering motor 44 and the injection motor 45,
so that control of the metering motor 44 and the injection motor 45
becomes easier. In addition, accuracy in controlling the above-described
back pressure is enhanced. Moreover, since the rotational force of the
injection motor 45 is resisted by the second spline nut 76, the metering
motor 44 does not receive the rotational force. Accordingly, the capacity
of the metering motor 44 can be decreased.
Since both the metering motor 44 and the injection motor 45 can remain in a
fixed position in all stages, the reliability of wiring for the motors and
the like is enhanced.
The drive unit of the injection apparatus having the above-described
structure can be separated into a motor assembly 81 and a drive shaft
assembly 82, as shown in FIG. 5.
The motor assembly 81 comprises the front support 14, the center support
15, the rear support 16, the front frame 41, the rear frame 42, the
metering motor 44, the injection motor 45, the first rotor shaft 56, the
second rotor shaft 57, the spline nut 62, the spline nut 76, and the like.
The drive shaft assembly 82 comprises the spline shaft 63, the bearing
retainer 64, the ball screw shaft 65, the ball nut 69, the spline shaft
71, the bearing box 72, and the like.
Accordingly, it is possible to separately manufacture the motor assembly 81
and the drive shaft assembly 82. After that, the metering motor 44 and the
injection motor 45 of the motor assembly 81 are driven to respectively
check their operations. After the checking, the drive shaft assembly 82 is
inserted into the motor assembly 81, and the front cover 13 and the rear
cover 17 are fixed to the motor assembly 81. The operation of the drive
unit is then checked.
Therefore, the motor assembly 81 and the drive shaft assembly 82 can be
maintained and managed separately.
Moreover, a bracket 86 is fixed to the rear cover 17, and an absolute pulse
encoder 85 is attached to the bracket 86 such that the absolute pulse
encoder 85 faces the end portion of the ball screw shaft 65. When the
injection motor 45 is driven in an injection stage so as to rotate the
second rotor shaft 57, the ball screw shaft 65 is rotated. During this
operation, the absolute rotational position of the ball screw shaft 65 is
detected by the absolute pulse encoder 85.
The position of the ball nut 69 is calculated based on the absolute
rotational position signal (absolute signal) output from the absolute
pulse encoder 85, and the lead of the ball screw shaft 65 (the amount of
movement of the ball nut 69 per revolution) so as to control the position
of the screw 22. In this case, a difference between the actual position of
the ball nut 69 and the position obtained by the calculation results if
play (backlash) exists between the ball screw shaft 65 and the ball nut
69. However, such an error can be mostly eliminated by using a pre-load
type ball screw device as the ball screw shaft 65. Since no mechanism for
transmitting drive power is interposed between the rotor 49 and the ball
screw shaft 65, accuracy in detecting the position of the ball nut 69 is
enhanced.
When a servomotor is used as the injection motor 45, an absolute pulse
encoder 85 for motor control is provided to feed back the number of
rotations of the rotor 49, pulses corresponding to the rotation of the
rotor 49, or the like so as to perform positional control, speed control
and the like for the screw 22.
In the present embodiment, since the rotor 49 and the ball screw shaft 65
are connected with each other via the second rotor shaft 57 and the
bearing retainer 64, difference in speed is not produced between the rotor
49 and the ball screw shaft 65. Accordingly, a signal (incremental signal)
representing the number of rotations of the ball screw shaft 65 can be
used to control the injection motor 45.
As described above, the use of the absolute pulse encoder 85 makes it
possible not only to detect the position of the ball nut 69 but also to
perform positional control, speed control and the like for the screw 22.
Accordingly, it becomes unnecessary to provide an encoder for driving the
injection motor 45. This reduces the cost of the injection apparatus.
The present invention is not limited to the above-described embodiments.
Numerous modifications and variations of the present invention are
possible in keeping with the spirit of the present invention, and they are
not excluded from the scope of the present invention.
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